174 
MEASUREMENT OF HIGH TEMPERATURES. 
[bull. 54. 
of the stem does not seriously increase the value of the stem error 
by increasing the number of interstitial pores, as will be seen. In this 
re-entrant bulb symmetry of form has been sacrificed in order that 
greater identity in the exposure of the air thermometer and of the 
thermocouple may be secured. Deville and Troost emphasize the 
desirability of spherical bulbs. But the expansion error is beyond 
question less serious than the calibration error, due to inequalities of 
temperature of bulb and thermo-electric junction. 
The pressure which bulbs of this kind can withstand at high red heat 
(1,000°) without deforming appreciably is certainly greater than am 
atmosphere, probably much more. Bulbs in which water is confined 
explode at high temperature with detonation and great violence. 
The results obtained with these forms of internally unglazed gas 
thermometers are called in question by Deville and Troost. Doubtless 
gas or moisture is forcibly retained in the pores of the porcelain. Hence 
the amount of gas in the bulb at low temperatures may be greater tham 
the pressure datum indicates. Again, in the constant-pressure method! 
of measurement the volume of the bulb, an essential part of the argument 
of the formula, can not be sharply defined. Hence the great desirability 
of performing the measurements with bulbs of porcelain, glazed, 
thoroughly both within and without. Such a form is given in Fig. 33. . 
Fig. 33. Inglazed spherical air thermometer bulb. Scale h 
Bulb c d ~k and stem e b are here distinct parts. They are calibrated 
separately, and prior to using are soldered together with feldspar andfl 
the oxy hydrogen blow-pipe. The bulb c d Tc ends in a short neck, cdr sX 
the part re* being just large enough to receive the stem ef snugly. J 
The canal c d, through which the bulb is glazed, eventually becomes' 
the prolongation of the capillary canal of the stem. 
Soldering together the bulb and stem is a difficult operation, and rails 
for much skill and patience on the part of the operator. The bulbs are j 
liable to breakage, and it is difficult so to solder the stem that the joint j 
may be hermetically sealed. I therefore feel justified in describing a 
machine of my own, by which such soldering can be effected. 
(828) 
